This invention relates generally to superconductors and in particular to superconductors made from a solid state reaction between two alloys.
Superconductors are usually compared in terms of critical current, I.sub.c, critical current densities, J.sub.c, and the critical temperature, T.sub.c. Critical current density values indicate the ability of the material to carry large currents. Values are obtained by dividing the critical current by the cross sectional area of the superconductor. The critical current I.sub.c, is defined as the maximum current passed through a conductor in a transverse magnetic field before a measurable voltage appears in the conductor. The critical temperature, T.sub.c, is the temperature at which a material achieves the superconducting property. Since the transition from "normal" to superconduction occurs over a temperature range, values for this parameter have been variously reported at the onstart of superconduction or at the midpoint of the temperature range. For the purposes of this application the critical temperature is the midpoint of the range and hence would be lower than the values reported in the other manner.
Intermetallic compounds having an A-15 crystal structure are known to be exceptional superconducting materials. This structure is also referred to as a beta-tungsten crystalline structure. One of the ways in which these compounds are obtained is by a solid state reaction between two alloys in a vacuum or inert atmosphere at an elevated temperature. These compounds are then used in a composite structure with the two reactant alloys. Three excellent examples of such superconductors are composites of VGa--V.sub.3 Ga--CuGa, NbSn--Nb.sub.3 Sn--CuSn, and VSi--V.sub.3 Si--CuSi.
The major difficulty associated with manufacturing supercondustors with A-15 compounds is fabricating them into usable configurations. First of all the A-15 compounds are extremely brittle and some of the reactant alloys also become brittle through work hardening. Another problem is the adverse effect impurities may have on the completed composite superconductor. Tightness of the bond between the two alloys producing the A-15 compound and grain size of the resulting A-15 compound are also important considerations.
These difficulties become particularly severe with the construction of multifilament wires. Since the construction of a multifilament wire begins with a multiple cavity matrix rod, the required cavity outgassing prior to the insertion of the filaments becomes especially difficult. If any cavity is not completely vacuated, growth of the A-15 compound is disrupted by diffusion barriers created by the remaining gases. The filament alloys have different hardness levels and different work hardening rates than the matrix alloys. Such differences can cause the filaments to cut through the matrix; so that, processing results in either the filaments extending through the surface of the matrix or touching each other. Another source of difficulty in processing multiple filament wires is the greatly reduced size of the filaments before and after processing. At best the size of the multiple filament is 1/10 that of the single filament and often is 1/100 of the single filament.
Processing superconducting single filament wires made from composites of VGa--V.sub.3 Ga--CuGa, or NbSn--Nb.sub.3 Sn--CuSn, or VSi--V.sub.3 Si--CuSi has been disclosed in U.S. Pat. No. 3,811,185 of Howe et al. issued on May 21, 1974 and entitled Method for Enhancing V.sub.3 Ga Thin Film Growth and in U.S. Pat. No. 3,926,684 issued on Dec. 16, 1975 of David G. Howe. Multifilament superconducting wires have been fabricated for numerous composite e.g., V--V.sub.3 Ga--Cu--Ga or Nb--Nb.sub.3 Sn--CuSn. However the aforementioned composites have not been fabricated into multifilament wire by these methods or by any other methods.